As mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand of secondary batteries has also sharply increased as an energy source for the mobile devices. Among them is a lithium secondary battery having high energy density and high discharge voltage, on which much research has been carried out and which is now commercialized and widely used.
One of the biggest problems caused from such a lithium secondary battery is a low safety. A possibility of the lithium secondary battery catching fire and exploding is strong in various situations, such as overcharge, external heat, and physical deformation. Consequently, there have been proposed various methods of preventing the overcharge, which is a cause of the combustion and explosion of the lithium secondary battery, and preventing the occurrence of an internal short circuit due to the physical deformation of the lithium secondary battery. However, a means for preventing the combustion and explosion of the lithium secondary battery is required in spite of the provision of such prevention measures, or a means for restraining the progress of the combustion of the lithium secondary battery is required when the combustion of the lithium secondary battery is initiated.
Generally, a method of mounting an element at the outside of a cell and a method of using a material contained in the cell may be used to prevent the combustion and explosion of a secondary battery due to the overcharge of the secondary battery. The use of a positive temperature coefficient (PTC) element and a current interruption device (CID) element using the change in temperature of the battery, a protection circuit using the change in voltage of the battery, and a safety vent using the change in internal pressure of the battery belongs to the former. The addition of a material that can change physically, chemically, and electrochemically depending upon the change in temperature or voltage of the battery belongs to the latter.
The elements mounted at the outside of the cell use the temperature, voltage, and internal pressure of the battery, with the result that a secure interception is accomplished. However, an installation process and an installation space are additionally required. On the other hand, the CID element is applicable to only a cylindrical battery. In addition, the elements do not effectively perform a protecting function the battery in connection with tests that require quick response time, such as an internal short circuit, a nail penetration, or a local crush.
As a method of using a material contained in the cell, there is a method of adding an additive for improving a safety to an electrolyte or electrodes. For example, a material that causes electrochemical polymerization under a condition, such as the overdischarge, may be added to the electrolyte such that the polymerization product of the material forms a passivation film on the electrodes upon the overdischarge or solidifies the electrolyte to restrain the abnormal operation of the battery. This chemical safety measure does not require an additional process and space, and is applicable to all kinds of batteries. However, the chemical safety measure does not guarantee the reliable operation of the battery. Furthermore, the performance of the battery is lowered due to the addition of the material.
Therefore, there is a high necessity for a technology to easily provide a battery with inflammability without affecting the operation of the battery and requiring a large space.